Mutarotation, definition

In contrast to other 2,5-anhydroaldoses (which exhibit mutarota-tion, possibly due to the formation of hemiacetals28), 2,5-anhydro-D-glucose does not show any mutarotation.27 The importance of this compound as a potentially useful precursor to C-nucleosides warrants a reinvestigation of the deamination reaction, and the definitive proof of the structure of the compound. The readily accessible 2,5-anhydro-D-mannose (11) does not possess the cis-disposed side-chains at C-2 and C-5 that would be required of a synthetic precursor to the naturally occurring C-nucleosides, with the exception of a-pyrazomycin (8). The possibility of an inversion of the orientation of the aldehyde group in 11 by equilibration under basic conditions could be considered. 

Mutarotation also manifests itself in the solubility behavior of lactose. When a-lactose hydrate is added in excess to water, with agitation a definite amount dissolves rapidly, after which an additional amount dissolves slowly until final solubility is attained. 

The work of Brpnsted and Pedersen (23) on the catalytic decomposition of nitramide and the kinetic studies of Lowry and Faulkner (24) on the mutarotation of tetramethylglucose led to the formulation of a more general viewpoint on acids and bases which logically showed that the hydrogen ion and hydroxyl ion were not the unique carriers of acid and basic properties. An acid was defined as any substance capable of donating a proton, and a base any substance capable of accepting a proton. In accordance with this definition (Lowry, 25 Brpnsted, 26), the following substances are typical acids and bases ... 

Finally, the mutarotation of tetra-O-acetyl-n-arohfno-hexose phenylosa-zone, of 4-deoxy-L-ffZyccro-tetrose phenylosazone, and of tetra-O-acetyl-D-lyxo-hexose phenylosazone can in no way be explained by cyclization, since in these cases there is definitely no possibility for ring formation to take place. 

The interpretation of these rate coefficients involves a complex line of reasoning based on several plausible assumptions. Briefly, the authors conclusions were as follows cis-trans isomerization and mutarotation occur at appreciable rates only in the presence of ionic chloride. Racemization may occur very slowly in the absence of ionic chloride, although the data do not permit a definite decision on this point. The most important reaction involves simultaneous cis-trans isomerization (with reversal of sign of optical rotation) and exchange with ionic chloride, probably by an 8 2 process. Since most events leading to racemization do not result in cis-trans isomerization, substi-... 

Many of the early studies were conducted with enzymes from fermentation, particularly invertase, which catalyzes the hydrolysis of sucrose to monosaccharides D-glucose and D-fmctose. With the introduction of the concept of hydrogen ion concentration, expressed by the logarithmic scale of pH (Sorensen, 1909), Michaelis and Menten (1913) realized the necessity for carrying out definitive experiments with invertase. They controlled the pH of the reaction medium by using acetate buffer, allowed for the mutarotation of the product and measured initial reaction rates at different substrate concentrations. Michaelis and Menten described their experiments by a simple kinetic law which afforded a foundation for a subsequent rapid development of numerous kinetic models for enzyme-catalyzed reactions. Although the contribution of previous workers, especially Henri (1902, 1903), was substantial, Michaelis and Menten are regarded as the founders of modern enzyme kinetics due to the definitive nature of their experiments and the viability of their kinetic theory. 

Solvents have a definite effect on the equilibrium composition. Thus, mannonic acid dissolved in acetic acid with 16 % of water shows a higher positive rotation than in water. The mutarotation is slower, but there is apparently a greater conversion to the 6-lactone than to the 7-lactone. The pH, temperature, and concentration also have an effect on the final equilibrium. 

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